Cryogenic liquids may be stored and transported in double-walled Dewar containers having a vacuum between the two walls for insulation purposes. To draw that vacuum, a manually operated valve fitting may be provided in the outer wall of the container for connection to a vacuum pump. With the valve open, the space is evacuated, following which the valve is closed to maintain the vacuum. In any valve system for such containers or for any gas pathway maintained under high vacuum, it is crucial that the seal between the movable valve member and its seat be tight to prevent leakage and contamination of the evacuated space.
Producing a reliable vacuum valve is difficult. Even the slightest defect in manufacturing may result in leakage at the valve seat. Additionally, the valve seat of even a perfectly manufactured valve can become worn, scratched or contaminated, thereby compromising the integrity of the valve seal. To solve these problems, some prior vacuum valves have a layer of material, in particular an O-ring made of organic material, between the valve member and the valve seat. However, organic O-rings outgas at low pressures, thereby introducing contaminants into the gas pathway. Moreover, organic O-rings become permeable over time and lose their sealing effectiveness. Thus, organic O-rings may not provide a reliably tight seal for containers or any gas pathway maintained under a high vacuum and may cause contamination. Although hollow, metal O-rings exist, they have not been used in the context of vacuum valves, i.e. as seals maintained under a high vacuum.